So after a little research:
The 2604 (that's what I've got) has 725in/lb torque, a mechanical clutch, and separate mode and clutch rings
The 2704 jumped up to 1200in/lb torque, went to a electric clutch (stops dead instead of ratcheting at the setting) and retained separate mode and clutch rings. It also has a larger thread on the chuck so that it doesn't directly replace the 2604 on a gen1 or gen2 clam plate.
The 2804 combines the mode and clutch rings (that's why its shorter than 2704), has electric clutch, and an improved side handle attachment.
I wish I could comment on country of origin, I suspect China for all 3.